MRI systems are commonly used to obtain an interior image from a patient for a particular region of interest that can be used to determine the health of the patient. MRI systems include a main magnet assembly for providing a strong uniform main magnetic field to align the individual magnetic moments of the 1H atoms within the patient's body. If the tissue is subjected to an additional electromagnetic field, which is tuned to the Larmor frequency, the 1H atoms absorb additional energy which rotates the net aligned moment of the 1H atoms. The additional magnetic field is typically provided by an RF excitation signal. During this process, the 1H atoms oscillate around their magnetic poles at their characteristic Larmor frequency, thereby emitting an NMR signal. When the additional magnetic field is removed, the magnetic moments of the 1H atoms rotate back into alignment with the main magnetic field. The NMR signal is received and processed to form an MRI scan or image. The MRI scan is most commonly based on the distribution of 1H atoms within the body. Bodily fluids have the highest density of 1H atoms, followed by soft tissues, then cartilage and then membranes.
If the main magnetic field is uniform across the entire body of the patient, then the RF excitation signal will excite all of the 1H atoms in the patient non-selectively. Accordingly, in order to image a particular portion of the patient's body, magnetic field gradients Gx, Gy and Gz in the x, y and z directions, having a particular timing, frequency and phase, are superimposed onto the uniform magnetic field such that the RF excitation signal excites the 1H atoms along a desired slice of the patient's body and unique phase and frequency information is encoded in the NMR signal depending on the location of the 1H atoms along the “image slice”. Gradient amplifiers are switched on to provide the magnetic field gradients Gx, Gy and Gz. The frequencies in the NMR signal come from different locations in the selected slice, while the signal strength reveals the density of the 1H atoms. The frequencies in the NMR signal also depend on the strength of the local magnetic field produced by the combination of the uniform magnetic field and the magnetic field gradients at the selected slice.
Typically, portions of the patient's body to be imaged are scanned by a sequence of measurement cycles in which the magnetic field gradients Gx, Gy and Gz vary according to the particular MRI imaging protocol that is being used. For each MRI scan, the resulting NMR signals are digitized and processed to reconstruct the image in accordance with the MRI imaging protocol that is used, many of which are well known to those skilled in the art.
The data acquired from the NMR signal is referred to as k-space data which is a two-dimensional data set in the case of 2D imaging. The k-space data provides frequency and phase information from which an MR image is produced via application of the inverse 2D Fourier Transform, for example. The manner in which the NMR signal is generated and sampled to provide the 2D k-space data is referred to as a k-space trajectory. Different k-space trajectories confer different properties on the reconstructed MR image.
Data acquisition for MR imaging can require a time period of several seconds to several minutes. During this time period, significant anatomical motion may occur. This is particularly true when performing cardiac, abdominal, joint, and interventional imaging. Without corrective action, this motion will produce artifacts that may degrade image quality.
To minimize motion-related artifacts, some techniques have been developed that synchronize the MR scan to the anatomical motion. To accomplish this synchronization, some measure of anatomical motion is obtained. Existing techniques attempt to infer motion through analyzing a variety of physiological parameters such as ECG waveforms (for cardiac-related motion) or diaphragm position (for respiratory-related motion). However, the drawback with these measures is that they may be unreliable in certain patient populations. For example, in patients with ischemic heart disease, arrhythmias may result in erratic ECG activity. Furthermore, some types of motion (e.g. joint motion) may not be easily related to any physiological parameter.
Other techniques that have been developed to minimize motion-related artifacts include the use of navigator echoes [1] in which two different types of MR data are acquired. The first data type is used to form the MR image. The second data type is used to assess and compensate for the anatomical motion that occurs over the course of MRI data acquisition. The second data type is typically acquired at regular intervals throughout MRI data acquisition, interleaved with the type 1 data acquisition. Data of the second data type is referred to as “navigator echoes”. Navigator echoes generally differ in two respects: the nature of the navigator echo, and the manner in which the navigator echo data is processed.
Most navigator echoes consist of data acquisition of a single line in k-space. This data is processed to provide a 1D projection of the anatomy. In some cases, additional localization in the remaining two spatial dimensions may be provided by the application of spatially selective radiofrequency (RF) pulses. Depending on the type of motion one is interested in correcting for, the navigator echo data may be acquired at a location remote from the anatomy of interest. For example, in cardiac imaging, navigator-echo data is typically acquired from the diaphragm to provide information about respiratory motion.
The 1D projections of the anatomy derived from the navigator echo data are processed to provide information on the nature of the anatomical motion. In many cases, this is accomplished by calculating the displacement between the 1D projections [2]. The displacement information is used to select the type 1 data that will provide an image with minimal motion artifacts. In most cases, type 1 data is selected based on the associated navigator echo possessing a minimal displacement.
However, there are some drawbacks with conventional navigator-based motion compensation techniques. First, because data is selected based on a minimum (i.e. not necessarily zero) displacement criterion, some residual motion may remain within the data. Under these circumstances, motion artifacts will be reduced, but not eliminated. In theory, the effects of motion could be eliminated completely by removing the navigator-echo-determined displacement from the type 1 data prior to image reconstruction. In practice, reduction in motion artifacts provided by this latter approach is limited [3] because the rigid-body motion calculated from the navigator echoes generally does not accurately describe the more complex (i.e. non-rigid-body) anatomical motion. This problem is exacerbated if the navigator is placed at a location remote from the anatomy of interest, such as in cardiac imaging where diaphragmatic navigators are used.